Preheat oven to 350 degrees and line two baking sheets with parchment paper.
In a medium bowl, cream butter and sugar together with an electric mixer on medium speed for about 2 minutes, until smooth and light colored.
1/2 cup butter, unsalted, 1 cup sugar
Add egg and vanilla extract and continue mixing for another minute.
1 egg, 1 tsp vanilla extract
In a separate bowl, whisk together the flour, cocoa powder, baking powder, and sea salt.
1 tsp baking powder, 1/2 cup Dutch processed cocoa powder, unsweetened, 1 cup flour, 1/2 tsp sea salt
Add flour mixture, half at a time and mix with a rubber spatula until just combined.
Using a 3 TBS cookie scoop, scoop dough and place 2 inches apart on the cookie sheet.
Sprinkle the tops of each cookie with sugar.
Bake for 10 minutes, rotating the pan halfway through.
Allow cookies to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.
Store cookies in an airtight container for up to a week or freeze for up to 3 months.
Notes
If you don't have Dutch processed cocoa powder and want to substitute natural cocoa powder, use 1/2 tsp baking soda in place of the 1 tsp baking powder. Make sure to read the cookie baking tips to get answers to common questions about baking cookies.